1. Field of the Invention
This invention relates to a silver plating technique, a high-speed silver plating technique and a silver strike plating technique and more particularly, to silver plating techniques which do not use any toxic cyanide.
2. Description of the Prior Art
Silver plating has been conventionally used for decoration and for dinner wares. Owing to its excellent electric characteristics, silver plating has wide utility in the field of the electronic industry as a material such as for switches, connectors and the like.
In the currently employed, practical silver plating baths, strongly toxic cyanide compounds are used in most cases accompanied by problems on safety operation or treatment of waste water. To avoid the problems, attempts have been made to use silver plating baths which are free of any cyanide compound, e.g. a silver nitrate-thiourea bath and a silver iodide-organic acid bath. Other types of baths have been proposed including a bath wherein triethanolamine is added to silver thiocyanate (Japanese Laid-open Patent Application No. Sho 54-155132) and a bath wherein sulfanilic acid derivatives and potassium iodide are added to inorganic or organic acid salts of silver (Japanese Laid-open Patent Application No. Hei 2-290993).
The silver plating baths using no cyanide compound impose less serious problems on toxicity and treatment of waste water on comparison with silver plating baths using cyanide compounds. However, when such silver plating baths are industrially used in practical applications, most of the baths are not satisfactory and there is some room for improvements particularly with respect to bath stability, uniformity in electrodeposition, critical current density, physical properties of deposit, and appearance. Especially, the baths have not been suitable for practical use in high-speed plating or strike plating. For instance, where silver strike plating is effected on a base metal matrix such as copper, nickel, or alloys thereof, the adhesion between the plated film and the matrix is not so good when using such a conventional non-cyanide plating bath as set out hereinabove. In addition, the solution is decomposed during use and the silver is liable to be reduced, with the tendency that the life of the bath becomes shortened.
The invention has been made to overcome the problems involved in these prior art techniques and has for its object a provision of a practical silver plating technique, a high-speed silver plating technique and a silver strike plating technique which have, respectively, such a performance as cyanide baths without use of any toxic cyanide.